TPREF is obsolete for the vast majority of users because Mark II systems were abandoned long ago.
TPREF is the reference time for Mark II tape changes. When automatic tape changes are specified, as is done by default, they will occur at times that are an integral number of tapelengths (default 4 hours) before or after this time. Since the only really valid Mark II tape length of 4 hours divides a 24-hour day evenly, no provision is provided to specify a day.